The organic EL device is a polycrystalline semiconductor device and, since it can emit light of a high luminance at a low voltage, it is used, for example, as backlight of a liquid crystal and is expected as a thin flat display device. However, the organic EL device has drawbacks. It is extremely weak against moisture. For example, the interface between a metal electrode and an organic EL layer peels off under the influence of moisture, the metal is oxidized and becomes high in resistance, and the organic substance itself changes in quality due to moisture, with consequent failure to emit light and lowering of luminance.
To solve such problems there have been proposed a method of molding the organic EL device with acrylic resin (JP 3-37991A), a method of shielding the organic EL device within a hermetically sealed case with P2O5 sealed therein (JP 3-261091A), a method of providing the organic EL device with a protective film such as a metal oxide and then hermetically sealing the device with use of a glass plate or the like (JP 4-212284A), a method of forming a plasma polymerization film and a photocurable resin layer on the organic EL device (JP 5-36475A), a method of storing the organic EL device in an inert liquid comprising a fluorinated carbon (JP 4-363890A), a method of forming a protective film such as an inorganic oxide on the organic EL device and then bonding a polyvinyl alcohol-applied glass plate onto the protective film through an epoxy resin (JP 5-89959A), and a method of sealing the organic EL device into liquid paraffin or silicone oil (JP 5-129080A). Recently, moreover, there has been proposed a method of adding a moisture absorbing material into a sealing resin and then laminating the sealing resin onto the organic EL device to protect the device from the influence of moisture.
However, all of the above conventional organic EL device sealing methods are unsatisfactory. For example, with a mere sealing of the device into a hermetically sealed structure together with a moisture absorbing agent, it is impossible to suppress the formation and growth of dark spots derived from the entry of oxygen and moisture. In the method of storing the device within a fluorinated carbon or silicone oil, not only the sealing step becomes complicated due to going through a liquid sealing step, but also an increase in the number of dark spots cannot be prevented to a perfect extent. Rather, there arises the problem that the liquid gets into the interface between a cathode and the organic layer, promoting the peeling of the cathode. Also in case of adding a moisture absorbing material to resin, there is a fear that the resin may absorb moisture before sealing, and thus the handleability is poor. Besides, there has been a case where, due to the absorption of moisture, the resin itself swells and cause peeling (JP 2007-284475A). There also has been proposed a method wherein, in order to eliminate a bad influence of moisture on the organic EL device, a moistureproof layer is formed separately from the sealing layer by adding a moisture absorbing agent comprising a metal oxide such as barium oxide or calcium oxide to a photocuring epoxy layer (JP 2001-237064A). In this case, the metal oxide mixed into the resin swells due to moisture and therefore, as the case may be, the organic EL device itself is destroyed.
In JP 2005-19269A, JP 2005-302401A and JP 2006-179318A there is introduced a technique for sealing an organic EL device wherein an organic EL device is formed on a glass substrate with use of a thermosetting resin composition, a resin composition is stacked so as to cover the whole surface of the organic EL layer, followed by lamination of a water impermeable glass substrate. In this technique, however, when effecting the lamination using the thermosetting resin composition described therein, there arise, under the influence of heat for curing the resin composition and the heat of reaction of the resin composition itself, the problem of deterioration of the organic EL device and the problem of peeling of a cathode from the organic layer caused by a stress strain at the time of curing. In case of epoxy resin being cured with an amine curing agent, there arise the problem of corrosion of the organic EL device through pin holes in a protective film and the problem of a lowering of transmittance of light caused by coloration of a cured composition.
To solve the problems related to the use of such a thermosetting resin composition, JP 2004-139977A introduces a technique for sealing an organic EL device with use of a photocurable resin composition comprising an acryl functional resin. However, the resin composition comprising an acryl functional resin has a drawback such that a chemical influence thereof on the organic EL device is serious and that a portion not irradiated with light is difficult to cure. In WO 05/019299 there is disclosed a formulation using an energy beam curing resin composition which comprises an epoxy functional resin. The sealing structure disclosed therein is a conventional hollowing sealing structure, which requires the use of a desiccant to ensure reliability, and with an optical loss being unavoidable because of a hollow structure. JP 2005-129520A describes the use of a flexible transparent sealing material. However, without bonding to a device interface, it is difficult to ensure a high reliability. In JP 2005-216856A it is described that a sealing layer containing a nanocomposite which comprises a layered inorganic material, a polymer and a curing agent is used for filling. According to the technique described therein, an applicable, thin film desiccant is formed from a conventional laminating type desiccant to ensure reliability. Thus, there is no bonding function for both upper and lower substrates, but it is required to separately provide a sealing or filling material to fill up a gap between both upper and lower substrates.
On the other hand, in JP 11-274377A there is disclosed a paste composition comprising a thermoplastic resin, an epoxy resin, a coupling agent, a silicon dioxide powder and an organic solvent, and it is described that IC and LSI chips are used for sealing directly. However, in the invention disclosed therein, importance is attached to stress relaxability (elasticity) of a cured composition. Although a description is found therein to the effect that the paste composition is superior in moisture resistance, no consideration is given to the amount of moisture contained in the system of the paste composition. Further, in case of using a two-component curing type epoxy resin, there are required operations for formulation and mixing, as well as associated equipment. Besides, since pot life exists, there has been a problem in point of workability.
In JP 9-176413A is disclosed a method of forming a transparent film with use of a maleic anhydride copolymer as a curing agent. However, it is impossible to effect lamination because the copolymer contains styrene. In JP 9-235357A and 10-135255A, imidazole is used as a curing accelerator together with an acid anhydride curing agent. Also in this case the application thereof to the organic EL device is impossible because of a high curing temperature. In JP 2003-277628A is disclosed a formulation using 2,4-diamino-6-[2′-methylimidazole-(1′)]-ethyl-S-triazine isocyanuric acid adduct for the purpose of imparting flame retardancy by the generation of an inert gas in curing. In this formulation system it is impossible to afford a transparent cured composition and so the use for sealing an organic EL panel is impossible. In WO 02/006399 and JP 2004-315688A there is disclosed a system using a phenoxy resin and a bisphenol type epoxy resin. But this system is not practical because of a low visible light transmittance or conspicuous coloration of a cured composition.
In the foregoing sealing method using liquid resin it has been a serious problem that bubbles are formed in the step of laminating the organic EL device and sealing glass to each other. It is very difficult to effect lamination without any bubbles throughout the whole surface of the display portion. The inclusion of bubbles has heretofore been a cause of lowering the life of the device. Moreover, in case of using liquid resin at the time of multichamfering from mother glass, masking is required for a portion not to be laminated, with consequent lowering of workability.
In connection with sealing using a liquid resin composition there is disclosed in JP 2008-59945A a method wherein a photocurable sealing material is applied dotwise at equal intervals throughout the whole surface to be laminated and sealing is done under alignment and gap adjustment. Also in this sealing method it is very difficult to make a uniform thickness control after lamination and the inclusion of bubbles is unavoidable. Besides, because of a low viscosity liquid permitting dot application it is necessary that a damming material of a high viscosity be provided around a substrate in order to suppress the spreading of the photocurable sealing material during lamination. In the case of a sealing material of a low viscosity, there is a fear of influence on the device such as, for example, the formation of dark spots in an initial stage.
In JP 2004-59718A and JP 2004-210901A there is disclosed an adhesive film or a heat-curable resin, using imidazole as a curing agent or a curing accelerator. These are both high curing temperature, causing a great damage to the organic EL device. In JP 2004-115650A is disclosed a formulation using a liquid imidazole compound, but in this system it is impossible to ensure a thermal stability in the coating step of forming into a sheet shape. Further, in JP 2004-292594A is disclosed a formulation comprising an epoxy resin, a phenoxy resin and a curing agent. In connection with this system, however, no reference is made to the fluidization start temperature, the amount of moisture and the amount of outgas generated, and the system is not suitable for use as a whole surface sealing material for the organic EL device.